It is a surprisingly little known fact that Gmail and Google Apps for Businesses customers can use instant aliases to better track potential spam mail originating from singing up for (free) services.
You “create” an instant alias by adding a +alias to your username.
For example, if your email address is email@example.com, you could use firstname.lastname@example.org when signing up for the shiny new freeby service.
This way, the next time you receive an unsolicited newsletter caused by signing up to this service, the sent to field will reveal its source!
Instant aliases work with all Gmail and Google Apps for Businesses accounts and require no prior setup. I’m using them all the time and they’ve helped me greatly to separate services which I can trust with my real email address from those, that hand them to spammers.